1. Field of the Invention
The present invention relates to airway pressure support systems and, more particularly, to a patient interface assembly for a pressure support system in which a pressure sensor and a valve of a flow control circuit are integrated into the patient interface assembly.
2. Description of the Related Art
Many individuals suffer from disordered breathing during sleep. Sleep apnea is a common example of such sleep disordered breathing suffered by millions of people throughout the world. One type of sleep apnea is obstructive sleep apnea (OSA), which is a condition in which sleep is repeatedly interrupted by an inability to breathe due to an obstruction of the airway; typically the upper airway or pharyngeal area. Obstruction of the airway is generally believed to be due, at least in part, to a general relaxation of the muscles which stabilize the upper airway segment, thereby allowing the tissues to collapse the airway. Another type of sleep apnea syndrome is a central apnea, which is a cessation of respiration due to the absence of respiratory signals from the brain's respiratory center. An apnea condition, whether OSA, central, or mixed, which is combination of OSA and central, is defined as the complete or near cessation of breathing, for example a 90% or greater reduction in peak respiratory air-flow.
Those afflicted with sleep apnea experience sleep fragmentation and complete or nearly complete cessation of ventilation intermittently during sleep with potentially severe degrees of oxyhemoglobin desaturation. These symptoms may be translated clinically into extreme daytime sleepiness, cardiac arrhythmias, pulmonary-artery hypertension, congestive heart failure and/or cognitive dysfunction. Other consequences of sleep apnea include right ventricular dysfunction, carbon dioxide retention during wakefulness, as well as during sleep, and continuous reduced arterial oxygen tension. Sleep apnea sufferers may be at risk for excessive mortality from these factors as well as by an elevated risk for accidents while driving and/or operating potentially dangerous equipment.
Even if a patient does not suffer from a complete or nearly complete obstruction of the airway, it is also known that adverse effects, such as arousals from sleep, can occur where there is only a partial obstruction of the airway. Partial obstruction of the airway typically results in shallow breathing referred to as a hypopnea. A hypopnea is typically defined as a 50% or greater reduction in the peak respiratory air-flow. Other types of sleep disordered breathing include, without limitation, upper airway resistance syndrome (UARS) and vibration of the airway, such as vibration of the pharyngeal wall, commonly referred to as snoring. Thus, in diagnosing a patient with a breathing disorder, such as OSA, central apneas, or UARS, it is important to detect accurately the occurrence of apneas and hypopneas of the patient.
Devices are known that attempt to detect apneas and hypopneas to determine in real time whether a patient suffers from a sleep apnea syndrome. Examples of conventional apnea/hypopnea detection devices are described in U.S. Pat. No. 5,295,490 to Dodakian; U.S. Pat. No. 5,605,151 to Lynn; U.S. Pat. No. 5,797,852 to Karakasoglu et al.; U.S. Pat. No. 5,961,447 to Raviv et al.; U.S. Pat. No. 6,142,950 to Allen et al.; U.S. Pat. No. 6,165,133 to Rapoport et al.; U.S. Pat. No. 6,368,287 to Hadas.
It is further well known to treat sleep disordered breathing by applying a continuous positive air pressure (CPAP) to the patient's airway. This positive pressure effectively “splints” the airway, thereby maintaining an open passage to the lungs. It is also known to provide a positive pressure therapy in which the pressure of gas delivered to the patient varies with the patient's breathing cycle, or varies with the patient's breathing effort, to increase the comfort to the patient. This pressure support technique is referred to as bi-level pressure support, in which the inspiratory positive airway pressure (IPAP) delivered to the patient is higher than the expiratory positive airway pressure (EPAP).
It is further known to provide a positive pressure therapy in which the pressure is automatically adjusted based on the detected conditions of the patient, such as whether the patient is experiencing an apnea and/or hypopnea. This pressure support technique is referred to as an auto-titration type of pressure support, because the pressure support device seeks to provide a pressure to the patient that is only as high as necessary to treat the disordered breathing. Thus, the effectiveness of treating a patient via an auto-titration type of pressure support system can depend to a great extent on the accurate detection of apneas and/or hypopneas.
Examples of conventional auto-titration pressure support system are disclosed in U.S. Pat. No. 5,245,995 to Sullivan et al.; U.S. Pat. Nos. 5,259,373; 5,549,106, and 5,845,636 all to Gruenke et al.; U.S. Pat. Nos. 5,458,137 and 6,058,747 both to Axe et al.; U.S. Pat. Nos. 5,704,345; 6,029,665, and 6,138,675 all to Berthon-Jones; U.S. Pat. No. 5,645,053 to Remmers et al.; and U.S. Pat. Nos. 5,335,654; 5,490,502, 5,535,739, and 5,803,066 all to Rapoport et al. All of these conventional pressure support systems, with the possible exception of U.S. Pat. No. 5,645,053 to Remmers et al., are reactive to the patient's monitored condition. That is, once a condition occurs that indicates abnormal breathing, the system alters the pressure support to treat this condition.
It is also known, however, that known pressure support systems have certain shortcomings associated with delays in the detection of breathing conditions and additional delays in therapeutically altering the pressure support in response to the breathing conditions. A pressure source such as a CPAP machine is typically connected with a patient interface device through the use of a long flexible tube that carries the flow of fluid such as breathing gases from the pressure source to the patient interface device. Such supply tubes typically have been 22 millimeters in diameter and six feet in length. A pressure variation within air typically moves at a speed of approximately one foot per millisecond and thus takes roughly six milliseconds to traverse a six foot length of tubing. When a patient experiences a condition that is indicative of abnormal breathing, the pressure variation that results from the condition takes roughly six milliseconds to be communicated from the patient interface through the six feet of tubing in order to be detected by a sensor situated on the pressure source (a pressure variation can also occur under normal breathing due to flow load changes). While the pressure source may be capable of altering its pressure very rapidly, the altered pressure likewise takes six milliseconds to travel from the pressure source and along the six foot length of tubing to be received at the patient interface. The total delay between the onset of the condition at the patient and the receipt by the patient of altered pressure support can thus be roughly twelve milliseconds. This delay is further increased by the system dynamics of the mechanism used to adjust the pressure.
In today's systems, the time delay just described is managed via control algorithms that assume a constant and consistent level of airflow resistance and (volumetric) compliance. This assumption in the algorithms severely limits airflow circuit designs to standard structures with relatively large cross sectional areas that provide generally constant and consistent levels of airflow resistance and compliance. The result is that masks are typically equipped with a relatively large and inflexible elbow near the nose, which is then connected to a relatively large and inflexible hose. For example, in a typical standard mask and elbow connected to a six foot hose, the total resistance at 60 LPM flow rate is 5×10−3 LPM/cm H2O (the hose)+16.7×10−3 LPM/cm H2O (the mask and elbow)=21.7×10−3 LPM/cm H2O. In addition, this resistance will typically vary by only about 1.5×10−3 LPM/cm H2O.
It is desirable, however, to be able to employ improved mask designs that may be defined by small and flexible tubing combined with unconventional airflow paths instead of the standard structures above. These features, however, will (by definition) have high resistance to airflow, and the resistance levels could change significantly throughout the course of the night (i.e. they are not constant and consistent). In addition, soft materials could have relatively high mechanical deformation which would lead to significant and variable volumetric compliance in the airflow circuit. There is thus a need for a solution that permits the use of such improved mask designs.